1. Field of the Invention
The present invention relates to a circuit arrangement for suppressing oscillations, and in particular to such a circuit arrangement for suppressing acoustic feedback in a hearing aid.
2. Related Application
The subject matter of the present application is related to the subject matter of a co-pending application of the same inventor, filed simultaneously herewith, entitled Circuit Arrangement For Recognizing Oscillations, Ser. No. 152,326.
3. Description of the Prior Art
The risk of acoustic feedback is present in electronic systems having a microphone and a speaker in relatively close proximity to each other. Hearing aids are particularly susceptible to such feedback effects because the acoustic transducers (microphones and earpieces, or receivers) are disposed only a slight distance from each other. This results in disturbing tones such as, for example, a whistling effect, to be experienced by the wearer.
In hearing aids, efforts have been undertaken to reduce the susceptibility of the hearing aid to feedback oscillation mainly by constructing the auditory channel, and by improving the sound-insulating capability of the plastic used to make the ear mold. Efforts have also been undertaken from an electrical standpoint, however these have been limited to clipping or shifting the frequency band, rather than attacking the oscillatory signal itself. For example, constant attenuation of the output signal is described in "A Feedback Stabilizing Circuit For Hearing Aids," by D. Preves in "Hearing Instruments, Vol. 37, No. 4, pages 34, 36-41 and 51.
Other circuits have recently been developed (for example as offered by RIM-Elektronik of Munich, West Germany, and the circuits described in U.S. Pat. Nos. 4,232,192 and 4,079,199) which recognize oscillations, and take steps to suppress the oscillations. Such circuits take the useful signal between the input transducer and a final amplifier, which precedes the output transducer, and amplify the signal with an additional amplifier. The amplified signal is compared to a threshold voltage in a comparator stage, and is supplied to a phase-locked loop (PLL). The PLL recognizes an oscillation when it occurs, and forwards a suppress signal to a notch filter, preceding the final amplifier. The notch filter suppresses the frequency range of the oscillation, or reduces the gain, as in the case of the circuit described in U.S. Pat. No. 4,079,199. As is known, however, when the input signal falls off, a PLL becomes unstable and drifts. The result of the drift is a periodic, acoustic noise signal.
Another oscillation-suppressing circuit is described in U.S. Pat. No. 4,091,236. In this known circuit, the filter used therein skips to a prescribed frequency when the oscillation ceases. A risk of drift when the input signal appears is also present in this circuit, however, because the circuit generates oscillation recognition signals as soon as input signals having irregular periods (the normal case) are no longer acquired.